In a Long Term Evolution (LTE) system, in order to obtain the information such as a cell ID and the like, and to complete time/frequency synchronization, User Equipment (UE) needs to implement cell search. The cell search is completed based on three kinds of signals: a primary synchronization signal, a secondary synchronization signal and a downlink common pilot, wherein the primary synchronization signal is used for Orthogonal Frequency Division Multiplexing (OFDM) symbol timing and frequency compensation, the secondary synchronization signal is mainly used for frame synchronization and cell ID group identification, and the downlink common pilot is used for system information demodulation.
According to the LTE standard (36.211: Physical channel and modulation), there are totally 168 secondary synchronization signals, respectively representing 168 cell ID (cell identification number) groups. The cell ID group indexes of the secondary synchronization signals are NID(1) (NID(1)=0, 1 . . . 167). At a transmitting end, a base station determines two short code sequences according to NID(1). The two short code sequences are sourced from a Golden sequence of which the length is 31, only the corresponding cyclic shifts of the Golden sequence are different, the specific cyclic shifts are related to NID(1), m0 and m1 respectively represent cyclic shift values corresponding to the two short code sequences. Specifically, the method for obtaining m0 and m1 according to NID(1) can be represented by the following formula (1):
                                                        m              0                        =                                          m                ′                            ⁢              mod              ⁢                                                          ⁢              31                                ⁢                                          ⁢                      m            1                    =                                    (                                                m                  0                                +                                  ⌊                                                            m                      ′                                        /                    31                                    ⌋                                +                1                            )                        ⁢            mod            ⁢                                                  ⁢            31                          ⁢                                  ⁢                              m            ′                    =                                    N              ID                              (                1                )                                      +                                          q                ⁡                                  (                                      q                    +                    1                                    )                                            /              2                                      ⁢                                  ⁢                  q          =                      ⌊                                                            N                  ID                                      (                    1                    )                                                  +                                                                            q                      ′                                        ⁡                                          (                                                                        q                          ′                                                +                        1                                            )                                                        /                  2                                            30                        ⌋                          ⁢                                  ⁢                              q            ′                    =                      ⌊                                          N                ID                                  (                  1                  )                                            /              30                        ⌋                          ⁢                                  ⁢                                            N              ID                              (                1                )                                      =            0                    ,                      1            ⁢                                                  ⁢            …            ⁢                                                  ⁢            167                                              (        1        )            
As shown in table 1 below, a relation table between NID(1) and m0, m1 can be obtained according to the formula (1).
Similarly, according to the 36.211 standard, one radio frame comprises 20 time slots; the secondary synchronization signals are transmitted on 62 sub-carriers (not comprising direct-current carriers) in the middle of penultimate symbols of the time slot 0 and the time slot 10. The specific transmitting method is that: the 62 carriers are divided into two parts, i.e. 31 carriers with odd indexes and 31 carriers with even indexes. In the time slot 0: the sequences corresponding to m0 are transmitted on the carriers with the even indexes, and the sequences corresponding to m1 are transmitted on the carriers with the odd carriers; in the time slot 10, the sequences corresponding to m1 are transmitted on the carriers with the even indexes, and the sequences corresponding to m0 are transmitted on the carriers with the odd carriers.
Via table 1, it can be seen that: m0 is always less than m1. Actually, in the receiving end, the terminal obtains the frame synchronization just by adopting the property. Suppose that, in the receiving end, the cyclic shift value of the sequence which is detected on the carriers with even indexes by the terminal is X, and the cyclic shift value of the sequence which is detected on the carriers with odd indexes by the terminal is Y, the step that the terminal obtains the frame synchronization and the cell ID group indexes according to X and Y is that:
1) if X is less than Y, it can be determined that the time slot of the secondary synchronization signals which are currently received is the time slot 0; and if X is greater than Y, it can be determined that the time slot of the secondary synchronization signals which are currently received is the time slot 10. If X=Y (such conditions exist because of interferences or channel fading), it can be determined that the currently-received signals are unreliable, and it is needed to wait for re-testing the subsequent secondary synchronization signals.
2) The cell ID group index NID(1) is obtained according to the formula (2):
                                          N            ID                          (              1              )                                =                                                    ∑                                  i                  =                  0                                                                                                                X                      -                      Y                                                                            -                  1                                            ⁢                              W                i                                      +            X                          ⁢                                  ⁢                              W            i                    =                                    Mod              ⁡                              (                                                      31                    -                    i                                    ,                  31                                )                                      .                                              (        2        )            
In an LTE-Advanced network, because of the introduction of Micro-cell, Pico-cell, Relay, Home NodeB and the like, the 168 cell ID groups in the existing standard cannot satisfy the requirements; it is very necessary to design a new mapping relation between the cell ID group index NID(1) and m0, m1 so as to support a greater number of the cell ID groups.
When designing the new mapping relation, it is very necessary to make the newly-designed mapping relation and the mapping relation in the existing standard be compatible, which is able to simplify the realization of the transmitting end and the detection complexity of the terminal. One simplest method is to adopt the former formula (1) and to expand the formula (1) to make NID(1) be greater than 167. However, when NID(1) is greater than or equal to 234, questions can be generated when applying the formula (1). For example: when NID(1)=234, according to the formula (1), it can be calculated that m0=22, and m1=0, here, m0 is greater than m1. But according to the former descriptions, if the terminal still implements the frame synchronization and obtains the cell ID group indexes according to the former methods, the frame synchronization can be failed, and the obtained cell ID group indexes are also incorrect; therefore the terminal cannot be normally accessed to the system.
From the above, the method for generating the secondary synchronization signals in the related technologies cannot support a greater number of cell ID groups.
TABLE 1NID(1)m0m10011122233344455566677788899910101011111112121213131314141415151516161617171718181819191920202021212122222223232324242425252526262627272728282829292930300231133224333534463557366837793881039911401012411113421214431315441416451517461618471719481820491921502022512123522224532325542426552527562628572729582830590360146125623663476458656966710678116891269101370111471121572131673141774151875161976172077182178192279202380212481222582232683242784252885262986273087048815892690379148925993610947119581296913971014981115991216100131710114181021519103162010417211051822106192310720241082125109222611023271112428112252911326301140511516116271173811849119510120611121712122813123914124101512511161261217127131812814191291520130162113117221321823133192413420251352126136222713723281382429139253014006141171422814339144410145511146612147713148814149915150101615111171521218153131915414201551521156162215717231581824159192516020261612127162222816323291642430165071661816729——————